The Ras GTPase has been implicated in about 20% of all human cancers. The regulation of signal transduction through Ras is complex and is not fully understood. Multiple nucleotide exchange factors can activate Ras, and other factors called GTPase activating proteins (GAPs) can inactivate it. One such GAP, p120, can associate with another protein called p190, which is a GAP for the Rac/Rho GTPase family, and can thereby link these two signaling pathways. The activation of the Rac and Rho GTPases is required for transformation by Ras, and mediates changes in the actin cytoskeleton. The goals of this project are: (1) to understand the mechanisms that regulate the Ras exchange factor, p140 Ras-GRF; and (2) to determine the function of the GTP-binding domain in the p190 RhoGAP. GRF is expressed predominantly in neurons, and may be the key regulator of Ras activity in the brain. GRF can be controlled by phosphorylation of Ser residues in response to muscarinic receptor activation. The first specific aims include: (a) identification of the phosphorylated Ser residues; (b) identification of the protein kinase responsible for agonist-dependent GRF phosphorylation; (c) the function of the pleckstrin homology, coiled-coil, IQ and Dbl domains of GRF in regulating phosphorylation; and (d) the role of GRF in neuronal signal transduction. The GTP-binding domain of p190 RhoGAP is required for morphological and protein kinase responses of cells to p190 expression. Specific aims in this section include identification of: (e) the mechanism by which the GTP-binding domain exerts these effects and the role of the interaction of p190 with p120 RasGAP; and (f) factors that regulate p190 GTP binding and hydrolysis, using the isolated p190 GTP-binding domain as a substrate. Overall these studies should lead to significant new insights into signaling through the Ras pathway, and may lead to new therapeutic targets for certain types of cancer.